Balance factor of contra rotating 90deg v twin

Nombre de messages : 4 Localisation : UK Date d'inscription : 15/11/2013

Hi

can anyone tell me what the optimum balance factor should be for a contra rotating v twin, such as a rotax 258. i have a project to make a similar engine using cr250 cranks and pistons. The tony foale pdf spells out what a rotax inline contra rotating twin should be, but i am confused about a 90 v twin.

any help gratefully received!

Jamie

Nombre de messages : 4 Localisation : UK Date d'inscription : 15/11/2013

the firing interval will be twingle - ie both firing at the same time

Nombre de messages : 4 Localisation : UK Date d'inscription : 15/11/2013

i got a response but it disappeared - sorry if i might have somehow made it disappear - i probably hit the wrong button if i did!

porter_jamie a écrit:: i got a response but it disappeared - sorry if i might have somehow made it disappear - i probably hit the wrong button if i did!

No worries Jamie. I wrote something and then erased it. You just happened to see it during the few seconds it was visible.
Both counter balance masses together should balance 100% of the resultant of both pistons when these are at Top Dead Center.
The essence is where to put these counter balance masses: 45° offset from the usual single-cylinder position.
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Nombre de messages : 4 Localisation : UK Date d'inscription : 15/11/2013

fantastic, thankyou.

If the cylinders were much larger would it be necessary to move away from both cylinders firing together and use a alternative firing sequence? If so how would you then balance the cranks?

Adam Armstrong a écrit:: If the cylinders were much larger would it be necessary to move away from both cylinders firing together and use a alternative firing sequence? If so how would you then balance the cranks?

Why are you asking Adam? Do you think the torque peaks of two big barrels firing simultaneously might destroy the clutch, the transmission or the tire? You may well be right. But I don't think you can correctly balance an out-of-phase tandem-twin. I'd rather choose a 90° V-twin with a single crankshaft and a 90° firing interval anyway.
By the way: I see you're from Newcatle. Shouldn't there be an extra t behind the a? Or maybe an s? Wink

Hi Frits, yeah that's what I was thinking. However I guess an alternately firing V twin with two cranks you'd generate a rocking couple effect.

Also, yes you're right. Thanks for the heads up.
I should really check my spelling!

Adam Armstrong a écrit:: Hi Frits, yeah that's what I was thinking. However I guess an alternately firing V twin with two cranks you'd generate a rocking couple effect.

That's right, although the rocking couple can be kept within limits if both con rods are close to each other.
Besides, you can fit a balance shaft, as Honda did in their two-stroke V-engines.
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Re your spelling: too bad you didn't change Newcatle into Newcattle Balance factor of contra rotating 90deg v twin 570102

Balance factor of contra rotating 90deg v twin 570102

Do you think therefore that you wouldn't be able to balance the RSW engine to run firing alternately?

Adam Armstrong a écrit:: Do you think therefore that you wouldn't be able to balance the RSW engine to run firing alternately?

Not contrarotating and vibrationfree without adding yet another shaft.

Thanks Frits. Whopping great big torque peaks it is then!

Adam Armstrong a écrit:: Thanks Frits. Whopping great big torque peaks it is then!

It took me some time but I think I've got my brain in gear now, Adam.
Tandem-twins and Armstrong, hmmm? You wouldn't have anything to do with the engine pictured below, would you?

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If you do, there may be a solution: set the crankshafts at 90° phase and skip the contrarotation. like the sketch below:
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If you use two 'rear' crankshafts, there would be room between their small primary gears for an intermediate coupling & balance gear.

Haha, I wish! Sorry Frits I didn't mean a tandem.
I was thinking of an engine with an arrangement like the Aprilia V twin you've shown above but enlarged to 1000cc.

Adam Armstrong a écrit:: I was thinking of an engine with an arrangement like the Aprilia V twin you've shown above but enlarged to 1000cc.

If the engine is not already in existence, I would advise against it. You should take a good look at a Honda NSR500V instead (its engine is shown further above). I love the simplicity of that bike (don't like the single-sided swing arm though).
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Nombre de messages : 2 Localisation : Italy Date d'inscription : 05/01/2016

Hello everyone :)
I know that this is not the latest topic on the forum but I have a question:
I know that you are talking about a 90° twin, sorry if this will be off topic, but chosing a 180° displacement (counterotating twin) will you get a better balance (with twin sparks)? And what about vibrations? I'm not considering that a 2stroke engine is usually placed in a bike or a kart and a 180° disposition can't be used for space problems
Thank you :)

Nombre de messages : 36 Localisation : Denmark Date d'inscription : 26/02/2013

Hello bioracing

A sketch of the system will further Your case.

Nombre de messages : 36 Localisation : Denmark Date d'inscription : 26/02/2013

Frits Overmars a écrit:: If the engine is not already in existence, I would advise against it. You should take a good look at a Honda NSR500V instead (its engine is shown further above). I love the simplicity of that bike (don't like the single-sided swing arm though).
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That is the nicest Bike I know;
apart from my MZ of course.
Next step in NSR500V development should have been to run both conrods on a single throw,cracked crank,one carburator,and a single united exhaust system.
And then allow lets say ccm550.The ugly fourstrokes were allowed to double up.

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